An electric power steering apparatus which energizes a steering apparatus of automobiles and vehicles in auxiliary power by motor torque energizes a steering shaft or a rack shaft with motor driving force in auxiliary power by a transmission mechanism such as a gear or a belt through a reduction gear. Such a conventional electric power steering apparatus is devised variously on control in a control apparatus of the electric power steering apparatus which energizes a steering apparatus by assist torque (steering auxiliary torque) so that a driver can smoothly conduct steering under any situations such as in high or low speed drive, or in running straight or at a curve, or in parking.
First, before introducing examples of specific conventional control methods, control over the electric power steering apparatus will be described in general. When the relationship between voltage and current of a motor, which are main control targets of the electric power steering apparatus, is expressed in an equation, it can be expressed by Equation (1).V=EMF+(R+s·L)·I   (1)Here, V is motor terminal voltage, EMF is motor back electromotive force, I is motor winding current, R is a motor winding resistance, and L is a motor winding inductance. S is a Laplace variable, which expresses d/dt. In addition, EMF is expressed by the following Equation (2).EMF=Ke·ω  (2)Here, Ke is a back electromotive force constant, and ω is rotor angular velocity.
(R+s·L)·I, which is the second term of Equation (1), is an electric element and has linearity. However, EMF of the first term is generated by the motor angular velocity ω, and greatly affected by nonlinear elements such as external forces from tires, and inertia and friction of mechanical elements of the electric power steering apparatus. Generally, the nonlinear element is hard to be controlled.
Here, feedback control (hereinafter, it is denoted as FB control), which is a typical control method, will be described briefly. Generally, FB control is that a target value, which is a certain control target, is controlled so as to match with a certain reference value. Typically, the difference between the target value and the reference value is inputted to a proportional integral circuit (hereinafter, it is denoted as a PI circuit), for example, for control. Then, the input of the PI circuit is a signal which includes all the mixed influences of variation of the reference value, disturbance and noise to the target value, fluctuations in parameters, and the like. Regardless how and which element among them exerts influence upon the control, it is such a quite simple control that whether the output matches with the reference value is determined and when there is no matching between them correction operation is conducted. Therefore, this pure FB control performs the correction operation only when an error exists so that the output is varied unstably near the reference value, which appears as a torque ripple of motor output in the electric power steering apparatus. A great torque ripple causes a problem that a driver feels uncomfortable in steering or with increased motor noise.
Here, there is an apparatus described in JP-A-2002-249061 as an example of the control apparatus using FB control (hereinafter, Patent Reference 1). The description will be described with reference to FIG. 5. Based on inputted car speed and steering torque, a current command value Iref is computed in a target current deciding section 120. More specifically, a steering torque detector 101 connected to a torque sensor, not shown, detects steering torque, a phase compensator 108 compensates a phase lag, and then its output is inputted to a steering torque controller 102. Furthermore, a car speed signal detected by a car speed detector 114 is inputted to the steering torque controller 102, and based on the both inputs, a torque value that assists steering torque generated by manipulation of a steering wheel by a driver is determined. Then, the torque value to be assisted is inputted to a motor current decider 107 to decide target current Iref.
Subsequently, actual current Iact of the output of a motor actuator 109 is detected by a motor current detector 111 and is fed back to a subtraction circuit 113 to which the target current Iref is inputted. Error between the target current Iref and the actual current Iact is computed, and it is inputted to a first current controller 103. Basically, output VdFB of the first current controller 103 drives the motor actuator 109 to control a motor 110. However, an auxiliary signal and an auxiliary control loop, described below, are added in order to smoothly conduct steering. First, as the auxiliary signal, disturbance voltage Vdist1 and disturbance voltage Vdist2, and back electromotive force Vb are compensated. Moreover, a disturbance voltage estimation observer 115 observes whether the motor actuator 109 makes output according to Vref that is a command value. The total disturbance voltage is (Vdist1+Vsist2+Vb), but the back electromotive force Vb is proportional to the steering speed, about 3 Hz at the maximum, whereas the disturbance voltage caused by brush vibrations and commutation ripples is 20 to 200 Hz. Thus, a highpass filter 116 is used to remove the back electromotive force Vb, and to extract only the disturbance voltage Vdist. The extracted disturbance voltage Vdist is inputted to a second current controller 105, and its output is added to VdFB by an adding circuit 112a, to compute motor drive command voltage Vref. The motor drive command voltage Vref is different from Vref, a basic control described above; it is a motor drive command voltage Vref that is corrected including the disturbance voltage which, compared with the basic control, enables to cope with various steering situations described above so as to smoothly manipulate the steering wheel. Accordingly, when the control apparatus in the configuration like this attempts to meet various steering situations, complicated control elements such as the highpass filter 116 and the second current controller 105 need to be added, resulting in a complicated control circuit.
Furthermore, Japanese Patent No. 2949183 (hereinafter, it is called Patent Reference 2) discloses an example of the conventional control apparatus of the electric power steering apparatus which compensates the back electromotive force described above. It will be described with reference to FIG. 14. In the drawing, control is carried out so that command value Ir computed in a torque sensor 100 mounted on the electric power steering apparatus is inputted to a control circuit 101, and a PWM control circuit 12 controls an inverter circuit 13 based on the command of the control circuit 101 to drive a motor 14. In the control circuit 101, motor current If detected by a current detection circuit 15 is fed back, error between the command value Ir and the motor current If is calculated, and the error is inputted to a PI circuit to compute the command value Vd. In the basic control method, the command value Vd is split into three phases (a-phase, b-phase, and c-phase) to be command values for the PWM control circuit 12.
Moreover, in the control circuit 101, ω is determined from revolutions N detected by a rotational speed sensor 102 mounted on the motor 14, back electromotive force E is detected from Equation (2), the back electromotive force E is added to the command value Vd to compute a new command value Vm, and the motor 14 is controlled based on the Vm. The compensation of the back electromotive force E allows smoother steering than the basic control method.
However, in the case of the scheme in which the motor revolutions are detected by using an encoder to compute the back electromotive force as shown in Patent Reference 2, the operation period of revolutions is set longer than the current control period in order to increase the resolution of revolutions to accurately compute the back electromotive force even in low-speed rotation. This becomes a factor to increase a lag in high-speed rotation. When the back electromotive force is small and the effect of the back electromotive force compensation is small as in low-speed rotation, the lag problem has small influence upon the back electromotive force compensation, but the lag causes a problem that the effect of the back electromotive force compensation is reduced when the back electromotive force is great as in high-speed rotation. More specifically, in high speed rotation, the back electromotive force compensation without delay is desirable.
The invention has been made in view of the circumstances. The invention relates to a control apparatus of an electric power steering apparatus. A first object of the invention is to compensate a nonlinear element of a motor of the electric power steering apparatus beforehand to linearize the motor. Furthermore, a second object is to conduct back electromotive force compensation with no lag by a control apparatus in which the back electromotive force of a motor is computed to compensate the back electromotive force for a control loop. By achieving the objects, a control apparatus of an electric power steering apparatus with less control error, stable controllability, small motor output torque ripple, a good steering feeling, and less motor noise is provided.